This invention relates to the desulfurization of gases to obtain an essentially completely desulfurized product gas suitable for discharge into the atmosphere. It especially relates to the desulfurization of stack gases and Claus plant tail gases.
Most Federal and State environmental regulations restrict the amount of sulfur components which may be discharged to the atmosphere. In California, for example, waste gases containing more than 10 ppmv H.sub.2 S or more than 500 ppmv of total sulfur compounds (calculated as SO.sub.2) may not be so discharged.
To meet such stringent requirements, two processes are presently being used commercially to treat feed gases containing several sulfur components, e.g., Claus tail gases. In both processes, the feed gas is first passed, as more thoroughly described in U.S. Pat. No. 3,752,877, through a cobalt-molybdate catalyst bed in the presence of H.sub.2 and/or CO to convert essentially all sulfur components to H.sub.2 S by hydrogenation and hydrolysis. The H.sub.2 S-containing gas thus obtained is then treated in either of two general ways. In one, the gas is passed through a solution comprising a sodium vanadate, sodium carbonate, sodium bicarbonate, and a chelating agent so as to convert the H.sub.2 S to liquid sulfur. Alternatively, the H.sub.2 S-containing gas may be passed through an absorbent comprising alkanolamines, whereby the H.sub.2 S is selectively absorbed. The rich absorbent containing dissolved H.sub.2 S is stripped and sent to a Claus plant, and the lean absorbent is recycled back to the absorber.
In both of the aforementioned processes, desulfurization is usually better than 95% complete. However, each has disadvantages. Using a solution of sodium vanadates for the conversion of H.sub.2 S to sulfur results in the concentration of thiosulfate ion therein, and, when the concentration reaches about 20 wt.%, the entire solution must be discarded, thereby resulting not only in the loss of expensive chemicals but also in the creation of a waste disposal problem. Using alkanolamines as an H.sub.2 S-absorbing medium also has disadvantages. Such absorbents are usually not as efficient as the solution of sodium vanadates for removing H.sub.2 S; moreover, the product gas withdrawn from the absorbent normally contains H.sub.2 S in concentrations higher than the 10 ppmv limit set by California regulations. Hence, this product gas must usually be thermally incinerated to convert the residual H.sub.2 S to SO.sub.2, a process which not only entails the consumption of fuel but also involves a substantial capital investment.
Other difficulties confront those attempting to desulfurize stack gas from power plants and the like. Such gases usually contain about 0.1-20 mol % of sulfur components, mostly in the form of SO.sub.2. To remove SO.sub.2 from such gases completely in the gas phase generally involves reduction of the SO.sub.2, usually to H.sub.2 S and then to elemental sulfur. Since this reduction cannot be accomplished in the presence of oxygen, which comprises 2-5 mol % of stack gases, it is usually necessary to consume all the oxygen in the stack gas with a fuel. But because stack gases are normally discharged at high volumetric rates, the fuel required to deplete the oxygen therein must be fed at a rate which is uneconomically high.